The present invention relates a high-stability device for the filtering and discrimination of an electric signal.
More specifically, the present invention concerns a device, which includes a filter section and a frequency discriminator, capable of detecting a variation in the frequency of an oscillator in relation to a given or center frequency, operating in such manner as to prevent, as much as possible the occurrence of output drift, even when the input signal is greatly affected by noise (that is, under conditions of low signal/noise ratio).
It is well known in the electronic arts how important it is to detect frequency variations in electric signals, for the purpose of using such information in operations such as generating oscillations and frequency tracking.
Known devices used for detecting such frequency variations typically consist of two basic systems: a filter section and a frequency discriminator.
The section allows only the frequencies that are close to a given or center frequency to pass through toward the discriminator, while the discriminator produces as output a signal, the amplitude of which is proportional to the frequency variation of the input signal in relation to the given or center frequency.
The discriminator generally is composed of two filters tuned, which are geometrically symmetrical with respect to the central frequency f.sub.0, are referred to herein below as f.sub.1 and f.sub.4 (FIG. 1B). and are used to produce output signals or opposite signs.
For the purpose of obtaining adequate band width performance, the filter section sometimes includes two resonant circuits, tuned on two frequencies, which are geometrically very close and symmetrical with respect to the center frequency f.sub.0 and referred to herein below as f.sub.2 and f.sub.3 (FIG. 1A).
For the proper operation of the system, the axis symmetry of the response curve of the discriminator must intersect the axis of the abscissae at a point, which corresponds to the center frequency f.sub.0, and its value must be zero at that point.
Furthermore, the axis of symmetry of the response curve of the filter must coincide with the axis of symmetry of the curve of the discriminator.
Non-compliace with such specifications causes incorrect identification of variations from the center frequency f.sub.0, particularly in the presence of input signals affected by noise. In order to meet such specifications and maintain reliable performance over time, it is necessary to build both the filter and the discriminator using circuits and components of high accuracy and stability (specifically, not affected by phenomena of thermal drift and aging).
Indeed, in the past, the low accuracy and stability of the traditional circuits composed of condensers and inductors caused extremely serious difficulties in building and operating devices made of such components.
A first attempt to overcome such obstacles was made by resorting to quartz-stabilized circuits, which, as is well known, have better properties with respect to accuracy and stability.
Even this solution, however, was found to be not without drawbacks, particularly in the design and final tuning stages, for the following reasons: the quartz circuits composing the filter and the discriminator must be tuned separately on frequencies f.sub.1, f.sub.2, f.sub.3 and f.sub.4 ;
the thermal and aging drifts in the various quartz circuits, although they are limited in extent, are uncoordinated with one another, thereby spoiling the alignment of the axes of symmetry; and
the low-frequency quartz components are bulky and of low reliability, especially when exposed to shocks and vibrations, which constitutes a limitation that often renders them unacceptable in designing the usable frequency range.